1. Field of the Invention
This invention relates to the removal and recovery of silver from an impure solution. More particularly, this invention relates to the adsorption of complexed silver ions from an impure solution and the recovery and reduction of the ions to metallic silver.
2. Description of the Related Art
Waste streams from photographic processes contain silver ions usually present in silver thiosulfate anion complexes such as AgS.sub.2 O.sub.3.sup.-1 or Ag(S.sub.2 O.sub.3).sub.2.sup.-3. These complexed silver ions are formed by the reaction of silver bromide with ammonium thiosulfate during the photographic development process.
Certain heavy metal ions are listed by the United States Environmental Protection Agency (EPA) as priority toxic pollutants. The heavy metal ions currently listed under EPA's priority pollutants list include silver. A discussion of the toxicity, health impacts and EPA's allowable limits for heavy metals such as silver can be found on page 2-3 in "Priority Toxic Pollutants: Health Impacts and Allowable Limits", edited by Marshall Sittig, published by Noyes Publications, Park Ridge, New Jersey in 1980.
Historically, very little attention was paid to the disposal of waste streams containing heavy metals, including those containing silver, due in part to lack of governmental regulation, and, more importantly, probably due to an overall lack of knowledge or appreciation by industry of the long term effects of industrial waste on people and the environment. Now, however, due both to the above referred to government regulations and corporate responsibility, much attention has been addressed to proper handling and disposal of waste materials classified as hazardous or toxic.
Not only has it become important to remove the silver from such waste streams for reasons of health, but the subsequent recovery of this valuable metal has also become of increased economic importance.
Three different processes have been utilized to remove silver from such waste streams. Conventional ion exchange followed by regeneration has been employed in large plants with discharges of 30 to 50 gallons per minute (gpm). An alternate regeneration or silver removal scheme may also be employed with ion exchange resins wherein an acid is added to precipitate the silver and the resin, after a number of cycles, is taken to a remote site and burned.
However, ion exchange is recognized as not being good at either meeting effluent discharge limits or exhibiting high silver recovery efficiencies. The primary reason is that the thiosulfate ion, S.sub.2 O.sub.3.sup.-2, is used as the counter ion and there is a high thiosulfate background in the stream. Poor equilibria is established and there is not good selectivity for the silver thiosulfate ion, Ag(S.sub.2 O.sub.3).sup.-3. Also, resin costs are very high, considering that the influent streams may contain 100 to 500 ppm Ag(S.sub.2 O.sub.3).sup.-3 which saturates a bed quickly.
Iron sponge cartridges have also been used, particularly in small (less than 50 gallons per week) shops, for silver removal. Silver replaces iron and forms a sludge which must be recovered extraneously. The best sponge performance is typically removal down to about 15 ppm, which is still too high for present effluent discharge limits.
The third technology which has been used for silver removal, and recovery is electrolytic reduction. Electrolytic cells do operate satisfactorily in reducing silver from solutions containing silver thiosulfate, but do not meet current effluent discharge limits with their discharge at about 100 to 500 ppm. Also, there is a high energy penalty for larger streams at lower silver concentrations.